cruise controll disengage when turn on headlights

What is your question?

I have a 91 f250 with a 5.8L with and over drive autom.tran.when you use the cruise controll it works good,but if you turn on headlight it disengages.I have checked the grounds under the hood and at rear on bed.They are good I clean them and put them back.It did not work. what could be the problem.